The present application relates to thyristor devices, and more particularly to gate-controlled bipolar semiconductor devices which achieve a conductive latchup state which can be turned off by a small gate signal.
Note that the points discussed below may reflect the hindsight gained from the disclosed inventions, and are not necessarily admitted to be prior art.
A thyristor is a four-layer solid-state structure which has long been an attractive candidate for high-current switches. Thyristors combine very high current ratings with very high voltage-withstand capabilities, which makes this class of devices the leading candidate for very high voltage switches and for handling very high power. For example, as of 2012, off-the-shelf packaged thyristors can withstand more than 10,000 Volts, and can switch more than 10 Megawatts of power in each unit. However, the basic thyristor structure cannot be turned off by just returning the “turn-on” terminal to 0 Volts, or even to a small negative voltage. Once it is turned on, it stays on for as long as it can draw a minimum holding current.
The basic thyristor structure can be thought of as a merged structure which combines a PNP bipolar transistor with an NPN bipolar transistor. Each of these bipolar transistors provides the base current of the other, so there is potentially a positive feedback relationship: the collector current in the NPN is the base current of the PNP, and the collector current of the PNP is the base current of the NPN. The gain of a bipolar transistor is normally stated as “beta” (β), which is the ratio of collector current to base current. In a thyristor, there will be positive feedback if the product of the two betas is greater than one (βNPN·βPNP>1). If this positive feedback relation is present, then, whenever the thyristor is turned ON, it will draw current up to the maximum the external terminals can supply, or until the bipolar devices reach saturation.
When this basic thyristor is OFF, the junction between the n-base and p-base regions will be reverse biased, and this condition blocks conduction. (The anode will be connected to a voltage which is more positive than the cathode voltage.) A depletion region, with a width depending on the applied voltage, will be present on both sides of this pn-junction formed by the base of the NPN transistor and the base of the PNP transistor. The two other junctions will be forward biased, but no current flows (other than leakage), since the reverse biased junction is present.
When the thyristor is ON, conduction is as follows. (Note that current is carried by both electrons and holes flowing in opposite directions, but current in the conventional sense only flows in one direction.) Holes will pass from the p+ anode region through the n-base region into the p-base region, and thence into the n+ cathode (where they will typically recombine with the majority carriers, which in the n+ region are electrons). Since the holes have positive charge, their movement means that current (in the conventional sense) flows from the anode to the cathode. Similarly, electrons will pass from the n+ cathode region through the p-base region into the n-base region, and thence into the p+ anode (where they will typically recombine with the majority carriers, which in this region are holes). Since the electrons have negative charge, their movement means that current (in the conventional sense) is opposite to the physical movement of the electrons, i.e. current flows from the anode to the cathode. Since current is carried by both electrons and holes, this thyristor is a bipolar (or “minority carrier”) device, and operates quite differently than unipolar (or “majority carrier”) devices, such as field-effect transistors, where current flows because of the motion of only one carrier type.
When a thyristor has been turned ON, it is electrically analogous to a simple junction diode, but with a lower forward voltage drop than a junction diode.
Issued U.S. Pat. No. 7,705,368 to Rodov and Akiyama, which is commonly owned with the present application, described a fundamentally new structure for a MOS-controlled thyristor (“MCT”). The present application provides improvements on the structures and methods disclosed in that patent.
Turn-on in an MCT is relatively simple, but turn-off is the more difficult challenge in this technology. The Rodov et al. patent describes (among other teachings) a MOS-controlled thyristor in which a mesa of n-type semiconductor between the cathode contact and the n-emitter/p-base junction can be depleted by a sufficiently negative gate voltage (applied to a trench gate). The gate trench extends down through the n+ emitter layer, and into, but not through, the p-base layer. The voltage on the gate electrode can cause depletion of the p-type material in these mesas, which “pinches off” the connection to the cathode terminal, and thereby interrupts conduction. However, a mesa width of less than the Debye length is required for the gate to turn the Rodov et al. device OFF. This gate-to-gate distance is typically 1 micron or less in such a thyristor device. For a discrete power device, this is very small and will result in a high manufacturing cost. The Rodov et al. device benefits from trench gate oxide thicknesses as small as 10 nm or so, but this further increases manufacturing costs and lowers the yield.
The thyristor devices of Rodov et al. have great advantages over IGBTs, but the Rodov et al. devices usually require that the gate voltage be pulsed positive to turn the device ON and negative to turn the device OFF. A power circuit used to control this type of thyristor device would therefore be quite different from the power circuit used to drive most IGBTs, in which the gate voltage is held constant while in the ON state, and returned to zero volts to turn the IGBT off.